JP6293675B2 - Wavelength tunable external cavity laser diode with GRSM for OCT - Google Patents

Description

FIELD OF THE INVENTION This invention is in the field of light sources, that is, the field of electromagnetic radiation sources in the infrared, visible and ultraviolet portions of the electromagnetic radiation spectrum. More specifically, the present invention relates to a light source, an optical coherence tomography apparatus, and a method for generating a light beam having a varying wavelength.

BACKGROUND OF THE INVENTION Adjustable light sources are useful for many applications. Among them is wavelength-swept optical coherence tomography (SS-OCT) in which the wavelength is swept back and forth between the upper and lower wavelengths at high speed. Other applications include DC adjustment (ie, setting the wavelength to the desired value and maintaining it for a certain time for measurement) or sweeping at various frequencies and / or different sweep characteristics, etc. I need.

  A variety of methods have been used to achieve a swept source (in general, the term “light” is used herein to refer not only to the visible range, but also to near and mid infrared and near ultraviolet radiation. Radiation, especially electromagnetic radiation in the range between 300 nm and 2000 nm). Among these are light sources in which a microelectromechanical system (MEMS) is used for wavelength adjustment. For example, as disclosed in WO 2010/11795 A1, certain gratings can be used in a Littrow configuration-where incident and diffracted rays are collinear (or coaxial)- It is caused by moving light rays incident on the grating, not by moving the grating.

  An object of the present invention is to provide a wavelength-tunable light source that can be adjusted over a wide range. A further object of the present invention is to provide a wavelength-tunable light source that has a small (instantaneous) output light bandwidth. Yet another object of the present invention is to provide a light source whose output light parameters vary only slightly over the wavelength range. Yet another object of the present invention is to provide a particularly compact light source.

  These as well as other objects can be achieved, at least in part, by the present invention as defined in the claims.

According to one aspect of the invention, the light source, which can be an external cavity semiconductor laser light source, in particular,
A semiconductor gain device operable to provide optical amplification;
A wavelength selective element comprising a diffraction grating, in particular a transmission diffraction grating, and
-A light redirector;
A gain device, an optical redirector and a diffraction grating are arranged with respect to each other to establish an optical resonator for the optical portion emitted by the gain device and diffracted by the diffraction grating;
The optical resonator is an external cavity laser resonator,
The light source can select the resonator radiation wavelength depending on the angle of incidence by varying the angle of incidence of the radiation circulating in the resonator onto the wavelength selection element (especially the diffraction grating).

  The light redirector includes a resonator end element through which light in the resonator travels. The resonator end element may be a mirror (one of which may be partially transparent to outcouple some of the light) or, in an alternative embodiment, the end element One may be a grating operated in reflection, such as the diffraction grating described above. In addition to the resonator end element, the light redirector may optionally include additional mirrors, prisms, lenses, etc., or other elements that affect the propagation of light.

  The concept of the invention is based on the strategy in which the wavelength λ is selected by tilting the input beam with respect to the wavelength selection element (in particular the diffraction grating): λ = fct (input angle). The wavelength selection element directs light beams of different wavelengths in different directions, so that the resonator condition is satisfied only for a specific wavelength range, while the light portion outside this wavelength range cannot reciprocate in the resonator.

  It can be specified that the diffraction grating is a transmission diffraction grating. In this way, increased resolution may be achieved while maintaining a very small form factor of the light source, since light travels twice in the transmission grating while propagating once in the optical resonator. This is because it can be diffracted. In this case, the optical resonator is established for the optical part emitted by the gain device and transmitted through the transmission grating. Alternatively, the diffraction grating can be a reflective diffraction grating.

  Furthermore, it can also be defined that the diffraction grating is static (stationary state). This is usually the case. The diffraction grating can be mechanically fixed with respect to the substrate, for example. It can be fixed in particular with respect to the light redirector and / or the gain device. In a static diffraction grating, the grating does not move during wavelength scanning. Very quick wavelength scanning may be achievable with static diffraction gratings and direction change devices provided to change the angle of incidence on the wavelength selective element (especially the diffraction grating). Due to the typical size and mass of a suitable diffraction grating, fast scanning can be achieved by moving the diffraction grating, although this can be achieved (more easily) by using a direction changer in other embodiments. Achieving speed is often not possible or at least quite difficult.

In many embodiments, the light source includes a direction varying device that can vary the direction of light incident on the wavelength selective element (which is normally stationary). The direction varying device may in particular deflect the light coming from the gain device so that its angle of incidence on the wavelength selective element (and especially the diffraction grating) of the light coming from the gain device can be varied. In one group of embodiments, the direction change device includes an optical deflector with a moving element that deflects light, such as an actuated mirror, such as a MEMS mirror, or an oscillating optical fiber. For more details on oscillating optical fibers and swept light sources including oscillating optical fibers, see R. Isago et al., “A wavelength swept with a 150 kHz sweep rate using optical fiber oscillation. laser with a sweep rate of 150 kHz using vibrations of optical fiber) ”, SPIE Proceedings 7004, 700410-1, which is incorporated herein by reference in its entirety). Depending on the application, the movable element may move in that it vibrates in a resonant manner, or a pseudo DC form, or the actual position is set substantially at all times by a control parameter such as a control voltage. May be moved in a “true” DC configuration. An alternative way of implementing the direction change device includes providing an electro-optic beam deflector. Under an electro-optic beam deflector, we are a device that includes a part of a material, the part of which varies the direction of propagation of light propagating in the material by the application of an electrical signal to the material Understand what you can do. There, especially solid materials are of interest, and in particular, said portion of material may be a non-linear optical crystal, such as a semiconductor structure or a KTN crystal. Typically, the material portion provides an entrance surface and an exit surface for light. Electro-optical beam deflectors based on semiconductor structures are described in more detail further below. More details on electro-optic beam deflectors based on nonlinear optical crystals can be found, for example, in the publication by Shogo Yagi et al. “KTN electro-optic deflection incorporating a mechanically free 150 kHz repetitive wavelength sweep. (A Mechanical-free 150-kHz Repetition Swept Light Source Incorporated a KTN Electro-optic Deflector), SPIE Proceedings Vol. 7889, 78891J-1, and “KTa _1-x Nb _x O ₃ crystal by J. Miyazu et al. 400 kHz Beam Scanning Using KTa _1-x Nb _x O ₃ Crystals, CLEO / QELS, paper CTuG5 (2010), which is incorporated herein by reference in its entirety. The above definition of electro-optic beam deflector further includes devices based on the Pockels effect (Pockels cell). At least according to current knowledge, electro-optic beam deflectors based on the Pockels effect are not suitable for applications where the size of the light source must be very small or particularly high scanning speeds are required. As such, such a deflector may be eliminated in the present invention.

  The wavelength selective element may include a reflective end surface that constitutes the end element of the resonator (and is therefore at the same time one of the light redirectors mentioned). In these embodiments, the wavelength selection element includes a diffraction grating (especially a transmission diffraction grating between two prisms) and includes blocks constituting a resonator end element, and may be in particular such a block. In most embodiments, the reflective end surface constitutes a mirror. Instead of a mirror, such a reflective end surface may be a further grating, in particular a grating in a quasi-Littrow configuration.

  In an alternative embodiment, the resonator end element is remote from the wavelength selection element, and a gas gap, such as an air gap, is disposed between the wavelength selection element and the end element. Further, in such alternative embodiments, the end elements may include end mirrors or may be retroreflective gratings.

  In any case, it may be advantageous to define that the wavelength selective element is located near the resonator end face. In particular, it can be defined that both the gain element and the optical delay (if provided) are located on the other side of the direction change device from the wavelength selection element.

  In these embodiments, the light transmitted through the grating is reflected by one of the resonator end elements, and may be considered to function in particular as a prism and possibly a periodic filter as described below. With the exception of the etalon, it returns to the grating without passing through any optical delay elements.

  The advantage of such a setting is that a small light deflector can be used in conjunction with the possible simplicity of the overall setting (the point of incidence on the light deflector is wavelength independent).

  Furthermore, many embodiments of the present invention provide increased resolution despite the simplicity of the double-pass conceptual shape inherent in the concept of placing a transmissive grating in a laser resonator.

  The “resolution” or “color resolution” of an element for a discrete wavelength of light is defined as R = λ / Δλ, where λ is the wavelength and Δλ is the smallest resolvable wavelength difference. For diffractive elements, the resolution limit may be determined by a Rayleigh criterion as applied to the maximum of the separated light part (diffraction maximum), ie one maximum is the other The two wavelengths are exactly resolved when located at the first minimum of.

  According to an alternative, in the case of a transmissive diffraction grating, a radiation deflection device (which can in principle be implemented like the direction-varying device described above) is arranged on the “back side” of the grating, ie a gain device. It may be arranged on the side of the transmission grating opposite the side. Thus, light transmitted through the transmissive diffraction grating propagating toward the radiation deflector is reflected back to the grating by the radiation deflector, which can be a movable resonator end element. The movable resonator end element then requires a sufficient size to reflect the light portion of the entire light source bandwidth.

  According to particular features from many embodiments, the wavelength selective element comprises an on-prism grating configuration (GRIMS). In this, the diffraction grating is arranged on a single prism, or in particular in the case of a transmission diffraction grating, between two prisms. Furthermore, settings with more than two prisms and / or more than one grating are possible. In GRISM, the dispersive power of the prism may increase the selectivity of the diffraction grating and / or can be used so that the non-linear relationship between input angle and wavelength can be flattened. Similarly, resolution that is generally not uniform over the entire spectral range due to different angles of incidence on the diffraction grating can be made flat (smooth) to show more uniform resolution over the spectral tuning range. In addition, the prism (s) can provide mechanical protection for the diffraction grating. It can be specified that the grating is a surface relief transmissive grating.

  In addition, it is possible to provide a prism with a curved entrance surface (especially attached to the diffraction grating, for example in GRISM). There, a concave prism shape is possible, but a convex prism shape is also possible. In the case of a concave prism shape, the entrance surface can draw a particularly circular shape. In the case of a convex prism shape, the incident surface can draw a circular shape, but it can rather draw a non-circular shape.

  It can be provided that the wavelength selective element comprises at least one curved diffraction grating, which curved diffraction grating can be the same or different in any case from the aforementioned diffraction gratings comprised in the wavelength selective element.

In a specific aspect of the present invention, the light source includes a second semiconductor gain device operable to provide optical amplification in addition to the first semiconductor gain device described above, and the direction changing device includes the first semiconductor gain device. The light amplified in the gain device and the light amplified in the second semiconductor gain device are received and arranged such that the direction of further propagation of the received light can be varied. In addition, a third or further semiconductor gain device can be provided. Such a light source may allow wavelength multiplexing. Spectrum (ASE amplified) the ASE of different semiconductor gain device will generally differ, they duplicate or may be non-overlapping. In the above aspect of the invention, the light source may be capable of simultaneously emitting and scanning two different wavelengths, in particular, clearly spaced wavelengths. The first and second semiconductor gain devices are included in one optical resonator or two optical resonators of the light source. In particular, two different embodiments with (at least) two different semiconductor gain devices are described.

  In a first embodiment, at least one beam splitter is provided to form at least two separate (partial) ray paths, and the first and second semiconductor gain devices are the at least 2 It is placed in a different ray path of two separate ray paths. In this case, both (or all) semiconductor gain devices can be placed in one and the same optical resonator, but two partially overlapping (or partially identical) optical resonators are formed. It is also possible to prescribe that. It is possible to use only a single direction change device (for light generated by the first semiconductor gain device and light generated by the second semiconductor gain device) and a single wavelength selection element It may be sufficient to provide only one of the single GRISM or other wavelength selective elements described in this patent application, for example. Separate optical paths in which different semiconductor gain devices are arranged can be considered as partial beam paths, where (at least generally) different wavelengths of light propagate. These separate optical paths can be considered as logically parallel ray paths. With one or more beam splitters, the separate optical paths are usually combined to convert them to two parallel optical paths, or more specifically to two coaxial optical paths, and corresponding parallel paths. Rays that are, or even coaxial, impinge on the direction changing device to achieve the aforementioned variation in the direction of light incident on the wavelength selective element. This first embodiment may be particularly suitable in the case of overlapping ASE spectra of these semiconductor gain devices.

  In the second embodiment, in addition to the first wavelength selection element described above, a second wavelength selection element is provided. And the second semiconductor gain device, and / or the further optical redirector, and the second wavelength selection element described above, wherein the additional second optical resonator is emitted by the second semiconductor gain device. To be established with each other. The second optical resonator is an external cavity laser resonator, and the direction changing device changes the direction of the light incident on the second wavelength selecting element to change the resonator emission wavelength to the second wavelength selecting element. Allows selection depending on the angle of incidence of the light above. In the second embodiment, the two optical resonators are both formed using the same direction changing device. The result is that it is possible (indefinitely) to produce an essentially synchronized wavelength scan for light of different wavelengths. This can be advantageous, especially in the case of optical coherence tomography applications, but can also be useful in other applications. This first embodiment may be particularly suitable in the case of (substantially) non-overlapping ASE spectra of semiconductor gain devices, but can also be applied in the case of (substantially) non-overlapping ASE spectra. .

  Utilizing one and the same direction change device for the simultaneous generation of light of different wavelengths is free from the problem of using two different direction change devices. These problems may be due to manufacturability issues, for example in the case of movable elements that deflect light (eg actuated mirrors such as MEMS mirrors), eg having the same (or at least sufficiently equal) resonance frequency It makes it difficult to produce or find two resonating and operable mirrors.

  In one group of embodiments (independent of the provision or non-definition of the specific aspect described above with additional semiconductor gain devices), the light source is a period (such as a Fabry-Perot etalon) placed in a laser resonator. Includes sex filter. The periodic filter helps to reduce the number of modes found in the external laser cavity and thereby further enhance the resolution. During a wavelength scan, for example, operation may jump from a sharply defined peak to a sharply defined peak.

  Such a periodic filter may be constituted by any other suitable means such as a Fabry-Perot etalon, or even an optical ring resonator. It may be placed on the gain element side of the direction change device according to the first option. According to a second option, the Fabry-Perot etalon may also be part of the wavelength selection element, or between the direction change device and the wavelength selection element or “behind” the wavelength selection element, ie It may be placed between the wavelength selection element and the resonator end face.

  In many embodiments, the light source further includes an optical delay, also referred to briefly as a delay. It is also possible to provide more than one delay unit. The delay unit may be disposed on the same wavelength selection element side as the side on which the gain device is disposed in the optical resonator. It may comprise a block of (usually solid) material, and a light path with a well-defined light path length is defined for the light propagating in the delay section generated by the gain device. The optical path length of the delay device may constitute at least 40% of the optical path length of the resonator, for example (the optical path length is calculated as physical length × refractive index). In general, the delay device includes a plurality of reflecting surfaces for back-and-forth reflecting a portion of light propagating in a block of material. Additionally or alternatively, the delay device may be defined at least in part by a waveguide.

  Such a delay may be part of a multi-planar resonator design, and the light guided in the resonator defines a first surface and a second surface that is different from the first surface. The deflection configuration deflects light circulating in the resonator from the first surface to the second surface. The light circulating in the resonator may then propagate on the first surface, for example in the delay section, and on the second surface in the wavelength selection element. In addition, light propagating in the resonator can additionally be defined to propagate in the third plane. The first and second faces can generally be oriented with respect to each other in any manner, and parallel orientation, and in some cases perpendicular orientation, can be particularly suitable. The same is true for the first side and / or the second side for the third side and possibly further existing side.

  A retarder made of a (usually) solid material with a refractive index n> 1 does not contribute to ray divergence as much as a smaller (refractive index) (usually gas) material is used, in addition, the same optical path length Less space is required to realize All of these can contribute to enabling the production of particularly small light sources. These effects apply not only to the delays that are probably present in the light source, but also to the prisms present there.

  The gain device can be, for example, a semiconductor optical amplifier (SOA). It is more particularly at approximately the middle of the optical path defined by the resonator (one movement from one end to the other end), for example at a position of 50% ± 25% of this optical path length. It can be arranged at a position of 50% ± 15% of this optical path length.

  The light source may be used for any application where a wavelength tunable light source is desired. According to an example, the light source may be used as a light source for wavelength swept optical coherence tomography (SS-OCT). In addition to the light source, the OCT apparatus may include an interferometer (or part thereof), and one or more detectors, along with additional elements such as k-clock, absolute wavelength trigger, scanning mechanism.

In addition to the light source, the invention further provides:
A portion of an interferometer that is in optical communication with the light source and is operable to combine a portion of the light generated by the light source and returned from the sample with a portion of the light generated by the light source and returned from the reference path;
A detector unit positioned to receive light so coupled from the interferometer.

In particular it adds
An optical element unit, which is suitable for focusing the light part originating from the light source on a focal point on the sample and performing a scan, said focal point and the sample being relative to each other; Moved.

The method of generating light rays with varying wavelengths is
Providing a wavelength selective element comprising a diffraction grating, in particular a transmission diffraction grating;
Amplifying light in a semiconductor gain device;
-Establishing an external cavity laser resonator for the optical part emitted by the gain device and diffracted by the diffraction grating;
Selecting the resonator radiation wavelength depending on the angle of incidence of the light on the wavelength selection element by varying the direction of the light incident on the wavelength selection element.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, embodiments and aspects of the invention will be described with reference to the drawings. The drawings are all schematic and are not scaled, and the same reference numbers refer to the same or similar elements.

An embodiment of an external cavity laser is shown. Furthermore, the embodiment of the wavelength selection element which comprises a cavity edge is shown. Furthermore, the embodiment of the wavelength selection element which comprises a cavity edge is shown. Furthermore, the embodiment of the wavelength selection element which comprises a cavity edge is shown. Fig. 3 shows an embodiment of a wavelength selective element with two transmissive gratings. Fig. 4 shows an embodiment of a wavelength selective element in which a further grating is operated under quasi-Littrow conditions. Fig. 4 shows an embodiment of a wavelength selective element comprising a separate resonator end constituted by a reflective grating. Fig. 4 shows an embodiment of a wavelength selective element with a Fabry-Perot periodic filter. Fig. 4 shows an embodiment of a wavelength selective element with a Fabry-Perot periodic filter. 3 shows an alternative embodiment of a periodic filter. FIG. 6 shows a top view of another embodiment of a laser. FIG. 4 shows a partial side view of another embodiment of a laser. 1 is a diagram of a light source system that includes an external cavity laser. FIG. FIG. 6 is a diagram of a wavelength selective element implemented as a GRIMS including a prism with a concave surface. FIG. 3 is a diagram of a wavelength selection element implemented as a GRIMS including a prism with a convex surface. FIG. 6 is a diagram of a wavelength selective element implemented as a GRISM plus a separate grating. FIG. 6 is a diagram of a wavelength selective element implemented as a GRISM plus a separate grating. FIG. 2 is a diagram of a wavelength selective element implemented as a GRISM with two transmission gratings and three prisms. FIG. 4 is a diagram of a wavelength selective element implemented as a GRISM with two reflective diffraction gratings. FIG. 6 is a diagram of a light source with a wavelength selective element implemented as a GRISM with one reflective grating. FIG. 3 is a diagram of a light source with a wavelength selective element implemented as a GRISM with a curved diffraction grating. FIG. 2 is a diagram of a light source with two gain devices that allow wavelength multiplexing. FIG. 2 is a diagram of a light source with two gain devices that allow wavelength multiplexing. It is a figure of the electro-optic deflector which can be used as a direction change apparatus. It is a schematic perspective view of an electro-optic light deflector. 1 is a side view of a semiconductor-based electro-optic beam deflector with one quantum well. FIG. 1 is a side view of a semiconductor based electro-optic beam deflector with two quantum wells. FIG. 1 is a front (face) or back (face) view of a semiconductor-based electro-optic beam deflector with a ridge waveguide structure. FIG. 1 is a front (face) or back (face) view of a semiconductor-based electro-optic beam deflector with a buried waveguide structure. FIG.

Detailed Description of Embodiments A laser light source 1 according to a first embodiment of the present invention is shown in FIG. The laser includes a semiconductor optical amplifier 11 (SOA) as a gain element (or gain device). The SOA is pumped by injecting current onto the pn junction. Electrically pumped SOA (and R-SOA) are known in the art and will not be described in further detail here.

  The optical delay unit 13 serves to increase the optical laser cavity length in a limited space. The collimator lens 12—which may be a gradient index lens (GRIN) or other suitable lens (eg, an aspheric or achromatic lens) or mirror (eg, an off-axis parabolic mirror) —is a cavity Serves to collimate the light circulating at the gain element 11. The delay unit 13 includes a block made of a transparent material such as silica, glass, or polymer with a reflective surface. The reflective surface may be completely reflective for total reflection and / or for the reflective coating applied. In addition to the reflective surface, the optical delay 13 further includes two surfaces that act as input and output coupling surfaces and are optimized for high transmittance. More detailed teachings regarding such optical retarders and their advantages can be found in WO 2010/11795 A1, which is hereby incorporated by reference in its entirety.

  The boundary of the laser cavity of the laser 1 is defined by an output coupling mirror 15 that transmits a defined portion from the cavity and a wavelength tuning configuration. The wavelength tuning arrangement includes a movable mirror 21 (or scanning mirror) as a scanning element, and a wavelength selection element 20 (particularly in the meaning of a very broad terminology) which is in particular the grating and prism arrangement 20 in the embodiment of FIG. Is a sex element). The grating and prism configuration 20 (also referred to as GRISM 20) includes two prisms 23 and 24 with a transmissive diffraction grating 25 in between them. The transmission diffraction grating 25 is, for example, a volume phase holographic grating (VPH grating; also referred to as a volume phase grating VPG), or any other type of transmission grating in this embodiment and all other embodiments, for example It may be a surface relief grating with a periodic structure or possibly a chirped structure. It is further noted that the diffractive grating can be implemented as a reflective grating, in particular in combination with a prism (reflective GRISM), see further below.

  Gain element side (in many embodiments it is also the delay part side: in many cases it is advantageous if the delay part 13 and the gain element 11 are on the same side of the scanning element 21 (movable mirror) The light propagating toward the wavelength selection element 20 is deflected by the movable mirror 21 and enters the first prism 23. From there it propagates to the diffraction grating 25. The first order (or possibly higher order) diffracted light portion of the second prism 24 propagates to a reflective end face 26 that simultaneously serves as the second end (in addition to the output coupling mirror 15) of the laser cavity. The light ray strikes the reflective end face 26 at a right angle to it, which propagates through the diffraction grating 25 and the first prism 23 in the same path and returns to the movable mirror 21.

  Instead of placing the SOA 11 and the collimation optics 12 on one side of the delay 13 and the scanning element 21 on the other side (all such positions and order and configuration are of course related to the optical path) instead of them Can also be arranged between the delay part 13 and the scanning element 21.

  Furthermore, beam shaping may not be achieved solely by the collimation optics 12, and in addition, as described in this patent application, additional elements or surfaces such as, for example, prisms or additional grating entrance surfaces may be applied thereto. You may contribute.

  As the movable mirror scans (in the embodiment shown, it is schematically indicated by a rotational movement about an axis perpendicular to the plane of the drawing), the incident point and angle on the transmission grating 25 change. As a result, the wavelength at which the condition that light enters the reflective end face 26 at a right angle is changed. As a result, the resonator wavelength (or resonator wavelength range) can be scanned by moving the mirror 21. The diffractive power of the grating and prism arrangement 20 may be increased by refraction at the prism end face of the grating and prism arrangement 20, ie, the face of the prism where light enters or exits the grating and prism arrangement 20. In the embodiment of FIG. 1, this applies only to the prism 23 (ie, the surface of the prism 23 depicted on the right in FIG. 1), and there is no reflective end face 26 and is not attached to the prism 24 (or It will be true for prism 24 if it is replaced by a mirror (at least not fully attached). Such refractive prism end faces “flatten” (smooth or more constant) the total dispersion of wavelength filtering achieved in the light source 1 to provide a more constant resolution for all wavelengths. Can contribute to). Typically, for relatively longer wavelengths, the resolution of the grating is higher than for relatively smaller wavelengths (which can generally be the opposite) and therefore (like prism end faces) The tilted surface can be arranged so that the elliptical ray shape on the grating 25 is initially achieved from a circular ray. The utilization achieved in this way of a larger number of grid lines can be adjusted for “flattening” the resolution.

  The advantage of a configuration in which the grid (grating 25) is passed twice is that the resolution (resolution) for a given number of irradiated grid lines is increased. In addition, due to the effects of the prisms and possible shapes with a less flat angle of incidence on them (see paragraph above), the resolution of the dispersive element 20 is typically about 70 ° or more It is flatter (more constant) over the spectral range than for a standard reflective grating in a Littrow configuration that is oriented to operate at a large angle of incidence. The flatter (more constant) resolution characteristics achievable in this way improves spectral performance over the case of similar reflective gratings (especially with no prism attached).

  A further advantage is that, despite the double optical path configuration, the dispersive element, in particular, is reflected by the end face of the second prism 24, as shown, or the reflective end of the resonator is of the GRISM configuration. It is still compact when placed in the immediate vicinity. The reflective end face 26 may be formed by a mirror (provided by coating or by attaching a prefabricated mirror, such as by gluing), or it may be an integrated or attached reflective grating.

FIG. 2 shows in more detail a GRISM dispersive element of the kind shown in the laser of FIG. In addition, the ray paths of rays of three different wavelengths, the minimum wavelength λ _min , the center wavelength λ _c and the maximum wavelength λ _max are shown. The refractive indices n ₁ and n ₂ of the first and second prisms 23 and 24 may be equal or different. It is also possible that n ₁ = 1 and / or n ₂ = 1, in which case one or both prisms may be omitted and replaced with air or protective gas. In many situations, n ₁ > 1 and n ₂ > 1 are preferred, and n ₁ = n ₂ is often chosen for practical (but not necessarily) reasons. The incident angle β on the first prism 23 is (for all angles occurring during the scan, and thus from λ _min to λ _max , so as to prevent further resonator modes due to the light portion reflected by the prism 23. May be chosen to be different from 90 °. In FIG. 2, such additional resonator modes do not exist for part of the angle under scanning operation; this is not the case only near λ _max . A broken line in FIG. 2 indicates a normal line on the interface between the material constituting the transmission type grating 25 and each of the first and second prisms. The angles γ and δ between each of the incident and (typically first-order) diffracted beams and their respective normals may be equal (symmetrical configuration with respect to the grating) or different (asymmetrical configuration). ). The angles at the diffraction grating refer to the surface normal of the grating, and the angles at the refractive surface are drawn in the figure to refer to the surface, but these angles are calculated in the calculation, for example Snell's When used in a formula, the angle referring to the surface normal must be taken, as will be apparent to any person skilled in the art.

Furthermore, the choice of the refractive index n ₁ and the set profile must be made such that at the maximum angle (maximum wavelength λ _max in the configuration shown) γ is below the critical angle for total reflection (TIR). TIR can occur when the incident prism 23 has a refractive index n ₁ greater than the grating structure (or grating layer) or when a bonding material such as an optical cement present between the prism 23 and the grating 25 is used in the prism. In this case, the refractive index is lower than 23.

  Another variation of the GRISM dispersive element 20 is shown in FIG. The incident surface (ie, the surface of the first prism 23 on which the light beam deflected by the movable element is incident) is tilted with respect to the orientation it has in the configuration of FIG. Such a tilt that exists in the orientation shown leads to an incident angle β> 90 ° (β is defined as shown in the figure) for all incident rays that occur during the scan, and within the laser cavity Residual reflections may be prevented or at least reduced, and may further help to make the resolution characteristics of the dispersive element 20 flat (ie, make the wavelength dependence of resolution more constant). The incident angle on the first prism 23 of the smaller wavelength light that satisfies the resonator condition is more obtuse than the incident angle on the prism 23 of the larger wavelength light that satisfies the resonator condition. As a side effect, the dispersion of the prism 23 increases the overall dispersion. The outer surface of the second prism 24 is approximately parallel to the diffracted beam path (and approximately perpendicular to the reflective coating 26), or arranged at a different angle so that the angle of incidence of the zeroth order diffracted beam is outside this. It is not perpendicular to the surface, again ensuring that unwanted laser modes are avoided.

FIG. 4 shows a further variation. The incident surface is tilted to achieve β <90 ° (for all incident angles occurring during the scan) and the incidence on the grating is relatively flat (eg, γ> 50 °). Again, this will make it possible to suppress residual reflections in the laser cavity. The configuration of FIG. 4 enhances the maximum resolution for λ _max due to the occurrence of beam shape deformation caused by the prism 23. For example, in the case of a round (cross-sectional) ray that strikes the entrance surface of the prism 23, the ray continues in the prism 23 with a slightly elliptical shape. The effect is more pronounced the smaller β (for higher λ).

  Further enhanced resolution (at the expense of reduced bandwidth and / or increased assembly size) is achieved by a multipath configuration with more than two paths through one or more gratings as shown in FIG. The Shown is a bulk (e.g., joined) in which three prisms 23, 24, 27 are attached to each other and a transmissive grating 25, 28 is between the pair of prisms 23, 24 and 24, 27, respectively. High assembly. The last prism 28 is again provided with a reflecting surface 26.

  An effect similar to that described for the configuration of FIG. 5 can also be obtained by placing two or more transmission gratings in a row and placing a reflector at the end, or transmissive GRISM and reflection. This can be accomplished by placing the mold configuration (eg, reflective GRISM as shown in one of FIGS. 2-4) in a row.

  FIG. 6 shows that the second grating 28 acts as a reflective grating instead of a transmissive grating (e.g. it may be a holography or surface relief grating), replacing the end reflector 26 of the previous embodiment. , Lattice 28 shows a variation arranged in a quasi-Littrow configuration.

  Configurations with more than one grid relax the requirements on the grid line density for a given desired dispersion, or increase the resolution by increasing the number of grid lines illuminated as required.

  FIG. 13 shows a wavelength selective element (specifically GRISM) comprising a prism 23 with a concave surface, including in particular a light incident surface having a concave shape. The incident surface here represents a part of the cylindrical (inner) surface. The shape can be adjusted to the direction change device, in particular to the relative position and orientation of the GRISM and to the concave prism surface. For example, in the case of a MEMS mirror as a direction change device 21 and with a circular shape with a radius R of the prism surface, the tilt axis of the MEMS mirror is the circular prism surface (on the surface on which the circle of radius R is drawn) Can be placed at a distance of R with respect to the axis of rotation of the vertical MEMS mirror. Thus, the use of possible refraction at the concave prism surface is not made for increasing and / or flattening the overall resolution or reducing the required maximum deflection angle of the direction change device 21. . It is sensible to specify that light rays striking the concave prism surface converge (slightly) (to the curved surface of the prism surface), because in that way the light rays (on the curved surface of the prism surface) This is because it can be specified that the light is collimated when hitting the lattice 25. Of course, it is also possible to place the tilt axis of the MEMS mirror (or the corresponding axis or point of a different direction change device) differently than described above, see eg the case shown in FIG.

  FIG. 14 shows a wavelength selective element (specifically GRISM) that includes a prism with a convex surface, in particular a light incident surface that describes a convex shape. The incident surface here represents a part of the cylindrical (outer) surface. The shape can be adjusted to the direction change device, in particular to the relative position and orientation of the GRISM and to the convex prism surface. Such a design may require achieving a greater (angular) deflection of the incident light and thus a greater scanning amplitude of the direction change device. On the other hand, however, it may be possible to allow the beam to be used on a direction change device, in particular smaller MEMS mirrors that are better suited for larger scan amplitudes and faster scan speeds, It is possible to focus on the corresponding MEMS mirror. The concave surface may depict a circular shape, but providing a non-circular (aspheric) shape allows the local curvature for one wavelength present at that location to be optimized, resulting in local For example, for further collimation, it is possible for a natural curvature to function as a wavelength optimized lens. In addition, the effects of aberrations can be minimized.

  Of course, curved prism surfaces such as the concave and convex described for the embodiments of FIGS. 13 and 14, respectively, will find application in other embodiments described in this patent application. be able to.

  FIG. 15 shows a wavelength selective element, in particular GRISM + separate (reflective) grating 87. Resolution is an important measure in the light source because it significantly affects the interference length of the generated light. With a wavelength selective element as shown in FIG. 15, improved (increased) resolution can be achieved because an increased number of lines of grating 25 and an increased number of lines of grating 87. This is because can be irradiated. This is due to grazing incidence on the grating 87 (typically above 45 ° relative to the grating normal combined with a diffraction angle relatively close to the surface normal). Initially using round rays, the rays draw an ellipse shape on the grid 87 and thus irradiate more grid lines on the grid 87, and more particularly in the grid 25, an increased number of lines, For example, irradiation is performed with respect to a configuration as shown in FIG.

  Effects similar to those achievable with the embodiment shown in FIG. 15 can also be achieved with the embodiment of FIG. Here, the transmission diffraction grating 88 is used in place of the reflection diffraction grating 87 in FIG.

  Similar effects can be achieved in the embodiment shown in FIG. There, the transmission diffraction grating 88 is attached to the prism 23 attached to the transmission diffraction grating 25. A further prism 29 is then (optionally) attached to the other side of the grating 88. Thus, here, the wavelength selective element comprises a GRISM comprising two gratings and three prisms, or rather a GRISM comprising two gratings and three prisms.

  Furthermore, with two reflective gratings, the above effect can be achieved as shown in FIG. There, the direction change device 21 is also drawn simply (in dotted lines). In this embodiment, the light traverses the GRISM prism on its way between the gain device and the direction change device 21, but the light is not diffracted by the grating while traversing the prism along its path.

  In FIG. 19, a fairly simple embodiment is shown in which GRISM is used as a wavelength selection device including one prism 23 and one reflection diffraction grating 25. The latter is in a Littrow configuration. Furthermore, the above effect of increased resolution is achieved here by grazing incidence on a grating combined with an optically denser medium (eg glass) attached to the grating. Typically, in such an embodiment, collimated light rays (e.g., generated by the generally optional collimation optics 12) enter the prism 23.

  FIG. 20 shows an embodiment in which a curved grid 89 is used. In this case, the ray between the direction change device 21 and the curved grating 89 is usually a diverging ray, and between the curved grating 89 and the grating 23 under quasi-Littrow conditions, the ray Is a collimated ray (as achieved by the curvature of the grating 89). Of course, it is also possible to define that the two gratings are both attached to the prism, in particular to one and the same prism (not shown in FIG. 20).

  In FIG. 7, the second grating 28 of FIG. 6 that is part of GRISM and is integrated (or typically joined) to the second prism 24 is shown. , Replaced with a separate traditional reflective grating 41. Instead of the reflective grating 41, it is conceivable to add further wavelength selection elements + reflectors, such as prisms, for increasing the transmission efficiency. This will reduce the benefits of increased resolution for better spectral bandwidth (and GRISM will become transmissive GRISM and no longer reflective GRISM), but such alternative configurations are special May be advantageous in certain situations. As a further alternative, it is conceivable to omit the prism 24 and / or the prism 23.

As a further variation, it is conceivable to choose both refractive indices n ₁ and n ₂ to be approximately equal to 1, so that the dispersive element is no longer GRISM, but merely a stand-alone transmissive grating.

  According to one group of embodiments, the dispersive wavelength tuning arrangement is combined with or includes a periodic filter. This is done periodically (k by reducing the number of modes found in the external cavity laser due to the combination of the dispersive element (GRIMS or other) and the well-selected spectral characteristics of the periodic filter. This results in an increased interference length (in space, not in λ space).

  In the embodiment of FIGS. 8 and 9, the periodic filter is a Fabry-Perot etalon combined with a wavelength tuning configuration.

In FIG. 8, the periodic filter is a Fabry-Perot etalon 51 integrated with GRISM. The end reflector 26 of the previously described embodiment is replaced with a separate reflecting surface 55 that forms the resonator end face. The Fabry-Perot etalon 51 has a partially transmissive mirror 52 and a further partially transmissive mirror 53 at the interface to the second prism 24. The transparent optical medium 54 between the two mirrors 52, 53 of the Fabry-Perot filter may have a refractive index n ₃ that may be equal to or different from one or both of the refractive indexes n _1, n ₂ of the prisms 23 and 24. Good. In special cases, the optical medium may be a gas, for example air. The etalon 51 is tilted with respect to incident light, in particular with respect to light incidence from the grating 25 and / or with respect to light incidence from the reflective surface 55.

FIG. 9 shows a modification of the principle shown in FIG. Again, the etalon 51 is tilted with respect to incident light, in particular with respect to light incidence from the grating 25 and / or with respect to light incidence from the reflective surface 55. Between the etalon 51 and the separate reflecting surface 55 is accompanied by a refractive index n ₄ that may be less than, greater than or equal to the refractive index of the transparent optical medium 54 of the tilted Fabry-Perot etalon 51 ( There are further blocks of solid (transparent) material 56. According to an alternative option, the space between the etalon 51 and the separate reflective surface 55 may be filled with a gas, for example air.

  In embodiments that include a periodic filter, the periodic filter need not be a Fabry-Perot etalon and / or need not be part of a dispersive configuration, i.e., it may further be in a laser resonator. It may be on the gain element side of the movable element 21.

  A simple schematic diagram of an alternative periodic filter 60 is shown in FIG. Light circulating in the cavity is guided to a first optical waveguide 61 that is coupled to an optical ring resonator 62. The amount of light that leaves the first waveguide 61 and is coupled to the ring resonator 62 depends on the wavelength of the light, and specifically depends on the wavelength of the light in a (wavelength) periodic manner. Depending on the requirements, the light transmitted through such a filter can be coupled to the optional second optical waveguide 63 by the light transmitted through the first waveguide 61 or via the ring resonator 62. It may be constituted by. In the former case, the first waveguide 61 constitutes a part of the cavity. In the latter case, one branch of the first waveguide 61, the ring resonator, and one branch of the second waveguide 63 form part of the cavity.

  Further embodiments of the periodic filter (or frequency comb) are possible and may be part of the laser resonator of the laser according to embodiments of the invention.

  Two embodiments suitable for simultaneously generating two rays of different wavelengths are shown in FIGS. In both cases, two separate gain devices 11a, 11b with different ASE (amplified spontaneous emission) spectra are provided. One identical directional device 21 (eg a movable mirror) is used in the light source. This can provide a unique synchronized simultaneous wavelength scan over different wavelength ranges.

  In FIG. 21, two logically parallel (partial) ray paths are formed, each including one semiconductor gain device 11a and 11b, respectively. This is achieved using beam splitters 16a, 16b, 17a, 17b, and the term "beam splitter" is meant in a very broad and general sense. They can all be conventional beam splitters, for example based on translucent mirrors, and beam splitters 16b and 17b can also be (conventional) mirrors, while beam splitters 16a and 17a are also WDM combiners (optical wavelength). It can also be a division multiplexing combiner). The output coupling mirror 15 is common to the light generated in either of the gain devices 11a and 11b. As in other embodiments, delay 13 (shown schematically using dashed lines) can be a valuable option. In the beam splitter 16a, light beams of different wavelengths are made parallel or coaxial, and two parallel or two coaxial light beams of different wavelengths hit the direction changing device 21 and enter the wavelength selection element 20 in the manner described above. The incident angle of light is varied (as shown by the dashed line in FIG. 21). Two separate optical collimating optical elements (one for each partial beam path) or one common such optical element can be provided.

  In addition, the beam splitters 17a and 17b and the output coupling mirror 15 are connected by one common output coupling mirror or by two separate output coupling mirrors (one for each partial beam path) or by the gain device 11a, It is also possible to replace by a mirrored (reflective) end of 11b. In that case, two partially identical (partially overlapping) optical resonators are formed.

The beam splitter can be polarization dependent or polarization independent.
In particular, if the maximum value of the ASE spectrum is quite far away, the higher order diffracted beam is transmitted to the first of the gain devices 11a, 11b (to accommodate the limited wavelength range in which the diffraction grating can operate). It can be specified to be used for light generated in the gain device.

  In FIG. 22, two optical resonators are formed, each including a gain device (26a / 26b), possibly a collimator (12a / 12b) and a wavelength selection element (20a / 20b)). Both resonators share a direction change device 21, for example a MEMS mirror. The gain devices 11a and 11b can be mirrored as shown in FIG. 22 (reflective end faces 26a, 26b), although they are separate mirrors, or even common mirrors, or others. It is also conceivable to provide an end face.

  From the above, it is clear how the embodiments of FIGS. 21 and 22 can each be generalized to more than two gain devices, and thus two or more different wavelengths simultaneously Allows the emission of scanned light. Wavelength multiplexing can be useful, for example, in an optical coherence tomography apparatus, but can also be useful elsewhere.

  In addition, the optical amplifier may be adjusted in time, for example, to reduce the ASE background while not lasing, and / or to flatten the output light in the spectrum, and / or for other reasons. Can do.

  Special features of a particularly compact embodiment of a laser, for example, are shown in FIGS. 11a and 11b. FIG. 11b shows the wavelength scanning configuration of the laser of FIG. 11a in a schematic side view.

  The first feature of the laser of FIG. 11a is that the gain element 11 is a reflective semiconductor optical amplifier (R-SOA). The cavity end mirror 15 is constituted by a partially reflective coating of the gain element 11. This first feature provides the advantage of less coupling loss in the cavity. On the other hand, embodiments with separate mirrors (as in the case of a non-reflective SOA as a gain element) are less prone to encounter spatial hole burning and generally have multiple mode operation (coupled laser cavities). ) Is not so sensitive.

  The second feature of the laser of FIGS. 11a and 11b that can be implemented independently of the first feature (ie, of these features, only the first, only the second, or both) Can be realized, or none of them can be realized) is a multilayer planar design. Light is guided in the first surface essentially through the gain element 11, the collimation optical element 12, and the delay 13, but the deflection arrangement 71 is light in a second surface parallel to the first surface. Where, generally, any mutual alignment or orientation of the first and second surfaces is possible. In a wavelength tuning configuration that includes a dispersive wavelength selection element 20 and a movable mirror 21 such as GRISM 20, light is guided in a second plane.

  In particular, the GRISM 20 may be placed on the delay unit 13. For example, it may be attached to it, for example with an adhesive.

  A schematic diagram of a light source including a laser 1 of the type discussed herein is shown in FIG. Outside the laser cavity of the external cavity laser 1 where the output coupling mirror 15 is a reflective coating of the gain element or a separate element, the following components are arranged:

  -A collimation arrangement 81 (e.g. a collimation lens or lenses) that, together with a gain element, functions to collimate the light into a fiber or optical feedthrough for light propagation (see below). The collimation configuration may be an aspheric lens or may include an aspheric lens. Further, it is conceivable to use a GRIN lens or a mirror. In principle, direct coupling to the fiber would also be possible.

  An optional optical isolator 82, preferably near the laser 1, to prevent any reflection back to the laser cavity.

  A beam splitter 83 that reflects part of the light beam to a wavemeter 84 (or k-clock), such as the wavemeter described in WO 2010/11795.

  A beam steering redirector 85; Such means are optional and may be realized in different ways, such as by a combination of prisms, wedges, mirrors, lenses, to form a telescope or the like. In general, such redirectors are passive, but can also be active components such as 1D or 2D MOEMS (micro-optical-electromechanical systems) for coupling efficiency adjustment. The closer the optical feedthrough is to the laser 1, the less important the optical beam steering that can be provided by such a redirector 85.

  A coupling lens 87 for coupling the light beam to the optical feedthrough 88; Depending on how the feedthrough is realized (which can be, for example, a waveguide or a window), a lens may or may not be necessary. Also, mirrors or other types of focusing optical elements can be used.

  -Fiber 88 as an optical feedthrough. The fiber can be cleaved or polished at an angle to prevent back reflections back towards the laser 1. It is also possible to combine a fiber with a lens, for example a GRIN lens glued (straight or angled) onto the fiber, the coupling lens 87 and the fiber feedthrough 88 together being a single component Would be considered.

  The above-described components may be used in combination, as needed, each alone or in any sub-combination.

  All elements described in the illustrated configuration are carried by a common substrate 90. The substrate may further be formed, for example, in a laser where different beam diameters are preferred for fiber coupling, to form different height optical axes possible above the substrate 90, or (of different thicknesses). It can be realized at several levels to accommodate the subassemblies on separate substrates. The substrate is preferably formed from ceramic and is relatively thick for mechanical stability, for example at least 0.5 mm and / or at most 10 mm, or at least 1 mm and / or at most 6 mm.

  The components held by the substrate are encased by a casing that provides mechanical protection. All components of the optical resonator can be arranged in the casing. The casing can be closed in a particularly sealed manner, and various environmental situations can be realized there, such as the presence of a gas / gas mixture or (some degree) vacuum. In addition to the optical feedthrough 88, the casing includes a plurality of electrical feedthroughs 91, particularly for control of the laser 1 and / or other one or more of the components.

  As the direction changing device 21, not only a MEMS mirror but also various elements, devices, or configurations such as a vibrating optical fiber or an electro-optic light deflector may be used. In the case of an oscillating optical fiber, the light beam from the gain element is supplied to the first end of the optical fiber, and the light exiting the optical fiber from the second end of the optical fiber (under variation in its direction) Propagating to the wavelength selective element, direction variation is achieved, for example, by moving (vibrating) the second end of the fiber. In the case of an electro-optic beam deflector, the optical path in the material is varied using an electrical signal applied to the material. For example, the application of an electric field by the application of a voltage to the material or the injection of charge carriers can cause variations in the way light propagates in the material, specifically incident light on the material and through the material. The properties of the material are altered in such a way that the angle between the rays that exit the material after propagation can be varied. This is illustrated schematically in a top view in FIG. 23, where α1 ≠ α2 indicates deflection. The direction of the arrow indicates the optical path from the light generating element, ie from the gain device, through the semiconductor structure 21 to the wavelength selecting element. The return path is the same but is not shown in this figure.

Light enters the device 21 (preferably comprising structured material) at a first interface 91 (front surface) under an angle α1. The angle of incidence (AOI) α1 may be 0, which would refer to normal (normal) incidence, or several degrees (eg, 1 ° -7 ° to minimize unwanted reflections at this first interface) °). An anti-reflective coating (ARC) can be applied to this first interface to suppress unwanted reflections. Within the device 21, light travels along the optical waveguide. At the first interface 91, the optical waveguide has an inclination angle of β1. Optimal coupling to the device 21 (from the free space / atmosphere), more specifically its waveguide, occurs when α1 and β1 follow Snell's diffraction law:
n _ext sin (α1) = n _eff sin (β1)
_Where n _eff is the effective mode refractive index in the semiconductor device 21 and n _ext is the material outside the device 21 (at the first interface), which is usually the atmosphere (or another gas or vacuum). Is the refractive index, and therefore the refractive index n _ext is near the unit element. For semiconductor devices, n _eff typically ranges from 3.0 to 3.5, depending on the design of the optical waveguide (such as waveguide dimensions and surrounding semiconductor material). Therefore, the inclination angle β1 of the optical waveguide at the first interface 91 is typically in the range of 0 ° to 2 °.

The light traveling in the optical waveguide enters the second interface 92 under AOIβ2 larger than the angle β1. Light exiting the electro-optic deflector 21 is diffracted under an angle α2 with β2 and α2 following Snell's law:
n _eff sin (β2) = n _ext sin (α2).

In a possible realization of the electro-optic deflector, the angle β2 is in the range of 10 ° to 20 °, depending on the value and range of n _eff . This means that the injection angle α2 is then in the range of 30 ° to 90 °, and larger angles may be particularly useful in the present invention.

  Importantly, the effective mode refractive index of the device 21 can be changed by applying an electrical signal, such as a voltage signal or a current signal, to the device 21. The voltage signal is usually applied by applying an electrical signal in the reverse direction, such as by connecting the positive (+) terminal of the power supply to the n layer and the negative (-) terminal to the p layer; Is typically applied by applying an electrical signal in the forward direction, such as by connecting the positive (+) terminal of the power supply to the p layer and the negative (-) terminal to the n layer. Changing the refractive index in the reverse direction via voltage is well known and can be described by the Franz-Keldish effect. However, this effect is generally more effective when changing light absorption by applying an electrical signal than when changing refraction. For practical realization of semiconductor-based electro-optic beam deflectors, the effective mode refractive index is therefore considerably varied by current injection (applying an electrical signal in the forward direction). Note that p-contacts and n-contacts are shown, for example, in FIGS. 24, 27 and 28, where p-contacts and n-contacts can in principle be considered interchanged with respect to what is described. Attention.

  In one example, a deflection variation angle Δα of 3 ° to 10 ° is desired, which means that changing the electrical driver signal will change the exit angle α2 by 3 ° to 10 °. The amount of refractive index change is usually limited by the design of the active area (where light propagates in device 21) and the applicability of electrical signals. However, an increased deflection variation angle Δα may be achievable by increasing the waveguide tilt angle β2. Consequently, the exit angle α2 may increase to its maximum possible value of 90 °. When the design parameters are appropriately adjusted, it is possible to achieve that a small refractive index change of 0.1 can already be sufficient to produce a deflection variation angle Δα of 5 ° to 10 °.

  The light exiting the deflector 21 is emitted, for example, towards the diffraction grating or diffraction grating and prism assembly described in this patent application. In addition to the propagation angle α2, the emitted light has a divergence angle described by the numerical aperture (NA) of the optical waveguide of the electro-optic deflector 21. This means that the emitted light has a light beam diameter that increases while propagating. For that reason, the emitted light should be collimated by suitable optical elements when used in combination with a diffraction grating, for example. This may be achieved, for example, by using a ball lens. Another option is the use of a concave diffraction grating to achieve ray collimation for diverging rays, for example as described for FIG. Electro-optic beam deflectors based on non-linear optical crystals that do not include waveguides typically do not exhibit increased divergence of the deflected beam.

  The active region of the semiconductor optical beam deflector 21 can be realized with a single quantum well (QW) having a thickness in the range of 2 nm to 40 nm, for example, or with a plurality of QWs having the same or different thickness values. . The optical waveguide can be realized, for example, with a ridge waveguide device, or a buried waveguide device, the latter typically being advantageous (considering interfacing with beam collimation and other optical components). A more circular mode profile may be generated.

Figures 24-28 further show possible semiconductor-based electro-optic beam deflectors.
FIG. 24 shows an active region 2.2 through which light to be deflected propagates, a top layer 2.4 that typically acts as a p-contact, and a bottom layer 2.3 that typically acts as an n-contact. 2 is a schematic perspective view of an electro-optic deflector 21 with

  FIG. 25 is a side view of a possible realization of the semiconductor deflector 21 showing the top layer 3.4, the active region 3.2 and the bottom layer 3.3. The active region consists of one quantum well 3.5 surrounded by two barrier layers 3.7.

  FIG. 26 is a side view similar to FIG. 25, but the active region includes two or more quantum wells, specifically one quantum well 4.5 and at least one other quantum well 4.6. Two or more quantum wells may have different stoichiometric compositions and / or different thicknesses.

  FIG. 27 is a front or back (face) view of a semiconductor deflector device 21 with a ridge waveguide structure, with a ridge 5.1, an upper layer 5.4, an active region 5.. 2 and lower layer 5.3 are shown. Typically, an electrical p-contact is realized on the ridge 5.1 and an electrical n-contact is realized on the bottom of the lower layer 5.3. The bottom of the lower layer may be a semiconductor substrate on which a semiconductor structure grown by epitaxy is grown, for example on a GaAs, InP or GaN substrate.

  FIG. 28 is a front or back (face) view of a semiconductor deflector device 21 with an embedded waveguide structure, with an upper layer 6.4, an active region 6.2 and a lower layer 6 that normally functions as an n-contact. .3. The waveguide, for example, etches through the active region to the lower layer 6.3, and then regrows another semiconductor material 6.5 having a lower refractive index, and includes an electrical contact layer, typically Can be formed by following another regrowth top (cap) layer 6.1 which acts as a p-contact.

Claims (26)

A light source,
A first semiconductor gain device operable to provide optical amplification;
A wavelength selective element comprising a diffraction grating;
-A light redirector;
The first semiconductor gain device, the optical redirector, and the diffraction grating are arranged with respect to each other so that an optical resonator is emitted by the first semiconductor gain device and diffracted by the diffraction grating. Established against
The optical resonator is an external cavity laser resonator;
The light source includes a direction changing device that makes it possible to select a resonator radiation wavelength according to an incident angle of light on the wavelength selection element by changing a direction of light incident on the wavelength selection element. Including
In addition to the first semiconductor gain device, a second semiconductor gain device operable to provide optical amplification, wherein the direction change device includes the light amplified in the first semiconductor gain device and the second semiconductor gain device A light source arranged to receive the amplified light in the semiconductor gain device and to change the direction of further propagation of the received light.   The light source according to claim 1, wherein the diffraction grating is a transmission diffraction grating.   The direction changing device is disposed between the first semiconductor gain device and the wavelength selection element, deflects light coming from the first semiconductor gain device, and allows variable incidence on the wavelength selection element. The light source according to claim 1, wherein the light source has an angle. The direction changing device is:
A light deflection arrangement with a movable element for deflecting light, or comprising a part of a material, the part of the material being adapted for the propagation of light propagating in the material by application of an electrical signal to the material The light source according to claim 1, wherein the direction can be changed.   The light source according to claim 4, wherein the movable element is a movable mirror.   The light source according to claim 4 or 5, wherein a part of the material is a semiconductor structure or a nonlinear optical crystal.   The light source according to claim 1, wherein the wavelength selection element includes the diffraction grating attached to a prism.   The light source according to claim 7, wherein the diffraction grating is a transmission diffraction grating disposed between the first prism and the second prism.   The light source according to claim 8, wherein the diffraction grating is attached to the first and second prisms.   The angle of incidence of radiation from the direction change device on the prism is such that light of a relatively larger wavelength at which the resonator condition is satisfied is relative to light of a relatively smaller wavelength at which the resonator condition is satisfied. 9. A light source according to any one of claims 7 or 8, which has a more obtuse angle than.   The light source according to claim 1, further comprising an optical delay unit disposed on the same side of the wavelength selective element as the first semiconductor gain device in the optical resonator.   The light source according to claim 1, further comprising a periodic filter.   The light source according to claim 12, wherein the periodic filter is a Fabry-Perot etalon included in the wavelength selection element.   The light source according to claim 13, wherein the Fabry-Perot etalon is arranged so that an incident angle of light circulating in the resonator on the Fabry-Perot etalon is different from 0 °.   15. The light source according to any one of claims 1 to 14, wherein the wavelength selective element includes a reflective end surface that constitutes an end element of the optical resonator.   The casing further includes a casing in which the optical resonator is disposed, and the casing includes an optical feedthrough for coupling light from the casing generated by the light source, and a plurality of electrical feedthroughs. The light source according to any one of claims 1 to 15.   At least one beam splitter is provided to form at least two separate beam paths, and the first and second semiconductor gain devices are arranged in different ones of the at least two separate beam paths. The light source of claim 1. The light source of claim 17 , wherein the at least two separate beam paths are formed in the same optical resonator or in partially overlapping optical resonators.   In addition to the first wavelength selection element described above, a second wavelength selection element is provided, the second semiconductor gain device, the aforementioned and / or further optical redirector, and the second wavelength selection. The elements are arranged with each other such that a further second optical resonator is established for the optical portion emitted by the second semiconductor gain device, the second optical resonator being an external cavity laser resonance The direction changing device varies the direction of the light incident on the second wavelength selection element to change the resonator radiation wavelength depending on the incident angle of the light on the second wavelength selection element. The light source of claim 1, wherein the light source can be selected. The spontaneous emission spectrum which has been amplified in the first and second semiconductor gain device is in duplicate, the light source according to any one of claims 1 to 19. The spontaneous emission spectrum which has been amplified in the first and second semiconductor gain device is a non-overlapping, the light source according to any one of claims 1 to 19. An optical coherence tomography apparatus, the light source according to any one of claims 1 to 21 , and
-Including a portion of an interferometer in optical communication with the light source;
The portion of the interferometer is operable to couple a portion of the light generated by the light source and returned from the sample to a portion of the light generated by the light source and returned to the interferometer via a reference path; And the optical coherence tomography apparatus further comprises:
An optical coherence tomography device comprising a detector unit positioned to receive the light so coupled from the interferometer; A method for generating light rays of varying wavelength,
Providing a wavelength selective element comprising a diffraction grating;
Amplifying light in a first semiconductor gain device and amplifying light in a second semiconductor gain device;
Establishing a first external cavity laser resonator for a light portion emitted by the first semiconductor gain device and diffracted by the diffraction grating;
Establishing a second external cavity laser resonator for an optical portion emitted by the second semiconductor gain device and diffracted by the diffraction grating, the first and second external cavity laser resonances The vessels are different from each other, or wholly or partially the same, and the method further comprises:
In the first and second external cavity laser resonators, by changing the direction of light incident on the wavelength selection element, the resonator radiation wavelength is selected depending on the incident angle of the light on the wavelength selection element Comprising the steps of: 24. The method of claim 23 , wherein the diffraction grating is a transmission diffraction grating. The spontaneous emission spectrum which has been amplified in the first and second semiconductor gain device is in duplicate, the method according to claim 23 or 24. 25. A method according to claim 23 or 24 , wherein the amplified spontaneous emission spectra of the first and second semiconductor gain devices are non- overlapping.

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